Lowering the Surface Energy and Boosting the Wear Resistance and Flexibility of Transparent Omniphobic Poss Coatings

Abstract

Transparent anti-smudge coatings have many potential applications. For applications on foldable smartphones these coatings should have simultaneously wear resistance and flexibility. These properties are normally mutually exclusive. These properties were achieved recently via the photoinduced cationic ring-opening polymerization of 3-glycidyloxypropyl polyhedral oligomeric silsesquioxane (GPOSS) grafted with the anti-smudge agent poly(dimethyl siloxane) (PDMS). While this was the first example of a hard and flexible anti-smudge coating, improvements to hardness/wear resistance, flexibility and surface energy were desirable. Herein, efforts to improve POSS-based omniphobic coatings are described. The first study involves the use of perfluoroalkyl groups in GPOSS to lower the coating’s surface energy. A systematic study is conducted by comparing the hardness, flexibility, and de-wetting properties of GPOSS coatings containing either -CF2CF3 or -CF2(CF2)4CF3 at various concentrations as the anti-smudge agent. Coatings containing the -CF2(CF2)4CF3 moiety are more effective at reducing the surface energy without significantly reducing the hardness. This coating can also withstand tens of bending cycles while maintaining its anti-smudge properties. In search of POSS coatings with a higher hardness and wear resistance, POSS consisting of two different polymerizable groups, specifically methacrylate (M) and glycidyl (G) groups, are prepared to give various GmMnPOSS compounds. In particular, m and n are 2, 4, or 6, respectively, and m + n = 8. These coatings are systematically studied to determine the optimized photoinitiator amounts and curing conditions before the optical transparency, hardness and flexibility are compared of the various POSS coatings. G2M6POSS offers the best balance between hardness and flexibility. To improve outward flexibility of G2M6POSS, a flexible porous poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) scaffold is first prepared. This porous scaffold is filled with G2M6POSS to produce a transparent and highly flexible coating that also maintains a high hardness. To endow the highly flexible G2M6POSS/PVDF-HFP coating with anti-smudge properties, a long chain perfluorinated polyether is covalently bound to G2M6POSS. The final fluorinated composite coating achieves a lower surface energy than is achieved with the perfluoroalkyl chains. These coatings can already be utilized for many applications and the insight gained from these studies will facilitate further development of hard yet flexible anti-smudge coatings.